Unidirectional microphone



Feb. 15, 1955 A. L. DVORSKY 2,702,318

UNIDIRECTIONAL MICROPHONE Filed Jan. 10, 1951 2 Shets-Sheet 1 INVENT ORALEXANDER L. DVORSKY ATTORNEY Feb. 15, 1955 DVORSKY 2,702,318

. UNIDIRECTIONAL MICROPHONE Filed Jan. 10, 1951 2 Sheets-Sheet 2FREQUENCY INVENTOR ALEXANDER L. DVORSKY United States PatentUNIDIRECTION AL MICROPHONE Alexander L. Dvorsky, Conneaut, Ohio,assignor to The Astatie Corporation, Conneaut, Ohio, a corporation ofOhio Application January 10, 1951, Serial No. 205,349

3 Claims. (Cl. 179-121) This invention relates to the art of convertingacoustical wave energy into electrical energy and more particularly topractical apparatus of an improved nature for accomplishing this generalobject in a directionallyfavored manner. Obviously, conversion systemsof this nature require an energy accumulator-for the application ofsuflicient mechanical power to a practical electrical generator and themost common and most economical type of collector is a pressed diaphragmof a relatively inelastic metal such as soft aluminum. To obtaindirectionality while using a simple structure having but a singlediaphragm it has been heretofore proposed to shift the phase of theacoustical waves impinging on the rear surface of the diaphragm in suchmanner that when the waves are propagated in a frontal direction, forexample, the effect of the phase shift or delay is to augment thedriving motion of the diaphragm while upon the waves being propagated inother directions the movement of the diaphragm is attenuated. It hasbeen heretofore proposed to accomplish this operation by enclosing therear face of the diaphragm with a chamber having a suitable acousticalimpedance in the opening thereinto for the passage of the acousticalwaves. Proper correlation of the volume of the chamber, the character ofthe impedance, and the linear distance between the front face of thediaphragm and the point of entry into the chamber results in aninstrument having fairly good uni-directional qualities.

It is, however, the primary object of the present invention to providean acoustical-electrical converter of the general character outlinedabove which is substantially improved as to practical structure andwhich has better characteristics in regard to uniformity of responsethroughout the whole of the audio frequency range. For permanency ofstructure and extended stability of operation as well as for economy inmanufacture I have found that the use of a sintered material has noequal. This sintered material may be readily fabricated in any desiredform by heating a compressed mass of pellets of the base material in asuitable furnace or induction heating apparatus as will be readilyunderstood by those familiar with the art. I preferably employ bronze asthe base material as the same is readily formed into a rigid strongstructure while yet allowing rather large voids to remain to give thedesired impedance throughout substantially the whole of the audiblerange of acoustical frequencies. In the simplest form of completeapparatus this sintered impedance material may be in the form of asingle disc which closes off the back of the chamber in the rear of thediaphragm while in other applications of the invention the material maybe employed for a portion of the housing or casing for the completemicrophone instrument. In the latter adaptation the housing or casingitself forms the required closed chamber to the rear of the diaphragmand in this manner an extremely economical and yet effectiveuni-directional microphone may be prouced.

Another object of the invention is the provision in a uni-directionalmicrophone of simple and practical design of an improved arrangement orincreasing the uniformity of response throughout the whole of theaudible frequency range. In uni-directional microphones of the prior artand of the kind employing but a single diaphragm and transducer with anacoustically impeded path to the rear surface of the diaphragm asexplained above the base or lower frequency response has been poor,particularly if the acoustical parameters have been ice properly chosenfor sharpness in directional effect. I

accomplish increased base responses while yet retaining sharpdirectionality by providing, in addition, to the normalimpedance'furnishing material at the rear of the closed chamber in backof the diaphragm, a secondary inlet into said chamber in the form of acurved tube of appreciable cross section which has a straightenteringend portion directed to the front of the microphone instrument.While the length of this tube may be sub stantial the same isnevertheless insuflicient, impractical apparatus, to delay theacoustical wave propagation to the extent necessary to properly augmentmechanical energization of the diaphragm and I accordingly provide asecond impedance element between the discharge end of the tube and theclosed space behind the diaphragm. This second impedance element mayconveniently take the form of a small aperture in a damped barrier wall.The advantage of making the conduction tube of large diameter is,however, that it supplies a sufiicient mass or volume of the Wave-frontcompressions to insure uniform and proper operation of the orifice inthe barrier.

The above and other objects and advantages of the invention will becomeapparent upon consideration of the following detailed specification andin the accompanying drawing herein there are disclosed certain preferredembodiments of the invention.

In the drawing:

Figure l is a vertical section through a microphone constructed inaccordance with the principles of my invention;

Figure 2 is a vertical section, on an enlarged scale, of a portion ofthe apparatus of Figure 1;

Figure 3 is a front elevation of the internal structure of the apparatusof Figure 1;

Figure 4 is a vertical section through a modified form of microphoneembodying the principles of my invention;

Figure 5 is a chart showing the performance characteristics of amicrophone constructed in accordance with Figure 1.

In Figure 1, reference numeral 10 designates a microphone stand on whichis mounted a housing 11 having a removable front cap 12 having slotopenings 13. Housing 11 is preferably a die casting and, as shown, it isprovided with a pair of forwardly extending bosses 14 to the outer freeends of which is detachably secured a casting 15 which has a small topaperture 16, an enlarged central aperture 17, and a lower compartment18. For supporting the housing 15 on the bosses 14 lugs 19 are castintegrally with the body member 15. An annular recess 20 is formed inthe front face of the casting 15 concentric with the aperture 17 andheld within this recess by means of a ring 21 is the diaphragm 22 which,as explained above, is preferably pressed out of a sheet of deadaluminum. A similar recess 23 is former in the rear face of the casting15 to receive the impedance disc 24 which, as stated above, ispreferably a sintered product made of bronze pellets. While the disc 24has an appreciable degree of porosity it is a substantial and rigidmember which I may utilize as the anchor for the fixed portions of thetransducer. For such transducer I preferably employ a piezo-electricalcrystal 25 and as indicated in Figure 2 portions of this crystal arerigidly cemented to the disc 24 through pads 26 while other portions ofthe crystal are engaged by a driving yoke 27. Yoke 27 has a forwardlyextending shank 27a which is suitably secured in a rigid manner to thecenter of the diaphragm 22. Disc 24 may also support terminal connectors28 and these in turn may be connected to the lead cord 29 through aterminal block or switch 30 housed in the casing section 11 and theremay be associated with this terminal block certain desired circuitmodifying components which are sometimes employed to further level outthe response of the microphone assembly. A wall 31 having an enlargedaperture 32 therein separates the compartments 17 and 18 of the casting15 and in accordance with the present invention I overlay the bottomsurface of the wall 31 with an inert sheet of soft metal or cloth 33having a small aperture 34 therein which registers with the aperture 32as shown in Figure 2. To enable the sheet 33 to be replaced or theaperture 34 to be enlarged during calibration of the instrument Iprovide a removable cover 35 for the compartment 18.

Cast integrally with the member 15 is a rearwardly disposed andapertured boss 36 which communicates with the compartment 18 andslidably connected onto this boss is a C-shaped tube 37 which curvesupwardly and forwardly of the compartment 17 to a straight portion 37awhich is directed to the front of the microphone above and ahead of thediaphragm 22. The entering end of the tube 37 is positioned as farforwardly of the diaphragm as is possible in a microphone casing ofpractical design. To support the upper portion of the tube 37 in thecasting 15 the tube is drawn through the aperture 16 from rear to frontthereof. The lower end of the tube 37 is, of course, supported on theboss 36.

By referring to Figures 1 and 3 it will be evident that the casting 15together with the diaphragm 22 and the disc 24 is widely spaced from theperipheral and back walls of the casing section 11 so that acousticallythe space behind the disc 24 may be considered as being entirely in theopen. The sintered metal disc 24 provides a porous or cellular passagefor the sound waves to the rear surface of the diaphragm and the porousor cellular mass has the effect of delaying the intermission of thewavefront compression as is well understood in the art. This, togetherwith the axial interval between the front face of the diaphragm and therear face of the disc and the volume of the compartment 17 serves todelay the application of frontal sound waves to the rear face of thediaphragm by approximately 180 to thereby accentuate or augment themovement of the diaphragm when sound waves are directed to its front.The phase displacement is such that when sound waves are propagated fromthe rear the compressions reach the front and back faces of thediaphragm at approximately the same time so that the activation of thediaphragm is at a minimum.

To make the microphone of practicable size the acoustical parameters aresuch that better directional and response characteristics are obtainedin the upper portion of the audible frequency range. This is desirable,of course, particularly in the avoidance ofacoustical feedback in publicaddress systems, for example, but for fidelity of reproduction I havefound that augmentation of the base response is also required. Iaccomplish this by the use of the tube 37 in combination with theresonance chamber 17 and the variable gate 33, 34. The long passage inthe tube 37 accounts for substantial shift in the phase of the lowerfrequency sound waves so that good directionality is achieved for theselonger waves, allowing adjustment of the size of aperture 34 to beavailable primarily for raising and lowering the response level of themicrophone at lower frequencies. In this manner I am able to provide amicrophone of simple yet practical construction which has improvedqualities as regards directionality and uniformity of response over thewhole audible frequency range.

By referring to Figure 5, where the upper curve shows the outputresulting from sounds originating at the front of the microphone and thelower curve the output resulting from sounds originating from the rearof the microphone, the persistency of the directional discrimination isat once apparent as is also the uniformity of response over the variousfrequencies.

In the embodiment of the invention shown in Figure 4 the microphonehousing is comprised of a principal rear section 40 and detachable frontcover section 41 which later is, in accordance with usual practice,formed with suitable grille openings to allow unimpeded entry of thesound wave. Casing section 40, however, is formed of sintered metalsimilar to the material used for the disc 24 in the first describedembodiment. Secured against: an annular shoulder 42 in the casingsection 40 by the cover member 41 is a perforated cup 43 which mountsthe fixed portions of the transducer crystal 44. A diaphragm 45 has itsouter edge rigidly secured in a peripheral bead of the cup 43 and thecenter of the diaphragm is rigidly secured to a driving yoke 46 for thecrystal 44. While the microphone of Figure 4 is inferior to themicrophone of Figures 1-3 as regards uniformity of frequency response itis, nevertheless, advantageous as being directional while yet being ofextremely simple and economical design.

In either of the embodiments herein specifically discloseddirectionality of response is achieved in an instrument which may bemade quite small in size, which is trim in design, rugged inconstruction, and permanent in adjustment and operation.

Since certain of the specific features of the microphone structuresdescribed above are conventional in the art and since many changes maybe made in the structures illustrated and described without departingfrom the spirit or scope of the invention reference should be made tothe appended claims in determining the scope of the invention.

I claim:

1. In a microphone having a casing with a rear wall formed of poroussintered metal providing acoustical impedance for the transmission ofsound waves into said casing, a diaphragm secured to and spanning theopen front end of said casing, a mechanical-electrical transducer housedin said casing and having a driving connection with a vibratile portionof said diaphragm, a closed chamber adjacent said casing, a removablebarrier member having an orifice therein separating said chamber and thespace within said casing, and an elongated tube having an entering endportion extending forwardly of the open face of said diaphragm and adischarge end connected with the space in said chamber.

2. Apparatus according to claim 1 further including a removable coverplate for said chamber whereby access may be had to said barrier member.

3. In a microphone having ahousing with a grilled front wall for theunimpeded entry of sound waves, a unitary member positioned centrally insaid housing and having a first chamber closed at its rear with a poroussintered metal wall providing acoustical impedance for the transmissionof sound waves and closed at its front by a diaphragm, amechanical-electrical transducer in said, first chamber having a drivingconnection with a vibratile portion of said diaphragm, a second chamberin said unitary member, means providing a restricted passageway betweensaid first and second chambers, and an elongated tube supported on saidunitary member and having its discharge end in communication with thespace within said second chamber, the entering end of said tubeprojecting forwardly of the open face of said diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS1,964,606 Thuras June 26, 1934 2,001,081 Thuras May 14, 1935 2,104,433Marshall Jan. 4, 1938 2,126,566 Marshall Aug. 9, 1938 2,444,620 Williamset al July 6, 1948 2,493,819 Harry Jan. 10, 1950 2,512,467 Olson June20, 1950 2,515,031 Black July 11, 1950 2,587,684 Bauer Mar. 4, 1952

